Eli5: Why do Diesel engines have a return line back to the diesel storage tank?
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Many gasoline vehicles also have return lines, just really depends on how the system is designed.
In most cases, there is a transfer pump in the tank which supplies fuel to the injector rail (or injection pump) at a set flow rate.
Since the engine obviously doesn’t need all the fuel, all the time, some systems have a return line to send the excess fuel back to the tank. This is generally controlled by some sort of pressure regulation device.
This prevents the pump from “dead heading”, and just pushing needlessly. Since many in tank pumps are actually cooled by the fuel they are pumping, the constant flow offered by the return line helps prevent overheating.
Return less systems may have a different style of pump, or a controller which varies the speed of the in tank pump depending upon load.
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